As American Girl only offers specific options in hair color, eye color, and skin tones (even within the Create Your Own Line), customization is the most popular way to have more unique dolls into a collection.
American Girl currently only offers three modifications to dolls outside of the Create Your Own line:
These are only available for 18" Dolls, in particular modern dolls.
The most common types of unofficial modification are:
Unofficial modifications may result in a doll being unable to be repaired by the American Girl Hospital.